The invention relates generally to the field of electromagnetic surveying of subsurface formations from within a wellbore. More specifically, the invention relates to structures for antennas for wellbore electromagnetic surveying that have azimuthally variant sensitivity.
U.S. Pat. No. 6,969,994 issued to Minerbo et al. and assigned to the assignee of the present invention describes one example of a well logging instrument including both longitudinal magnetic dipole and tilted magnetic dipole antennas so that, among other functions, distances between a wellbore and a formation boundary can be estimated in the presence of anisotropy and relative dip (angle between the wellbore axis and the direction of layering of the subsurface formations.
Tilted magnetic dipole antennas can take up a substantial length along the drill collar on which they are mounted, particularly if the collar is a relatively large diameter. It is known in the art to use collocated saddle coils and longitudinal magnetic dipole antennas to achieve tilted magnetic dipoles, however, such use requires combining signals from the separate coils. One disadvantage of a connecting two separate coils, in series particularly, is that relative phase and gain changes between the coils can be significant. Such changes are typically not compensated for when making both upward and downward measurements.
There exists a need for an antenna coil structure for electromagnetic well logging that can provide a selected magnetic dipole moment orientation without the need to connect separate coils or to use tilted coils to provide a selected oblique dipole moment direction.